1. Field of the Invention
This invention generally relates to apparatus and method for forming a laid fibrous article, characterized by a non-stepwise gradation in basis weight across its surface, such as is useful as an absorbent pad for applications such as disposable diapers, sanitary napkins, and the like.
2. Description of the Prior Art
In the general practice of forming fibrous web materials as laid fibrous articles, it has been common practice to utilize a fibrous sheet of cellulosic or other suitable fibers which is fiberized in a conventional fiberizer or other shearing device to form discrete fibers, which then are entrained in an air stream and directed to a foraminous forming surface whereon the fibers are deposited to form a pad of fluff, i.e., a nonwoven mat of randomly arrayed fibers containing substantial interstitial void space and being highly compressible in character.
The forming surface utilized in such systems typically is constructed as a wire or screen grid and typically employs pneumatic flow means, such as vacuum suction apparatus, to define a differential pressure zone on the forming surface and impose a pressure differential thereon, whereby the air in the air-entrained fiber stream is passed through the openings or perforations in the screen or grid of the forming surface. The use of vacuum suction to draw the air-entrained fibers stream of the forming surface, with passage of the air component through the forming surface, is highly efficient and lends itself to high-speed commercial operations.
Nonetheless, the use of such "vacuum deposition" systems for forming air-laid fibrous articles has the corresponding disadvantage that it is frequently difficult to quickly remove the laid fibrous article from the forming surface. This difficulty arises because formation of fibrous webs formed by vacuum-suctioned deposition of the fibers onto the forming screen produces a highly mechanically interlocked array of fibers, due to the many cross-over points of contact on a given single fiber relative to other fibers deposited therewith. Such interlocking produces a desirable structural integrity for the fibrous material, but the varied cohesive character of the fibrous mat hinders its removal when the entire web is attempted to be removed from the forming surface. This problem of removability is further exacerbated by the fact that the screen and grid elements commonly employed for constructing such forming surfaces become filled with the mechanically interlocked fibers around the strands or wires of the surface itself which increases the adhesion of the fibrous web to the forming surface.
Additionally, when a negative pressure, viz, vacuum suction is applied to the forming surface, the same must be terminated or otherwise overcome to break the vacuum force retaining the fibrous web on the forming surface. Therefore, in order to overcome the problem of removing laid fibrous webs deposited by vacuum suction on forming surfaces, it has been conventional practice to employ a pressurized stream of air, either as a side stream of the gas flow passing through the forming surface or as a separate gas stream, to break the vacuum imposed in the forming step upstream of the removal step and thereby forcibly remove the laid fibrous web from the forming screen or grid, typically onto a take-off conveyor or other similar means.
In the prior practice of forming laid fibrous articles, various means have been proposed in the art for providing the fibrous article with gradations in basis weight across the surface thereof. Such gradations can, for example, enhance the efficiency of the fibrous article in end usages, such as disposable diapers and sanitary napkins. Various means thus have been proposed for producing longitudinal gradations of basis weight, i.e., in the machine direction, and for providing transverse basis weight variations, i.e., in the cross-machine direction. In many instances, however, the aforementioned removability problem is rendered more severe by the fact that the additional basis weight imparted to the article provides more resistance to removal by a gas nozzle or other imposition of a pressurized gas stream. Accordingly, it would be a substantial advance in the art to provide means and method for forming a laid fibrous article characterized by a gradation in basis weight across the surface of the article, but which is readily removable from the forming surface subsequent to the laying of the article thereon.
U.S. Pat. No. 4,388,056 to Lee et al. discloses an apparatus for continuously forming an air-laid fibrous web, comprising a laydown drum having a circumferentially segmented annular-shaped plenum comprising a multiplicity of circumferentially spaced transverse plenum segments, and a partially masked foraminous laydown surface having oppositely contoured, cyclically undulating side edges defining cyclically circumferentially spaced relatively wide masked and relatively narrow masked transverse areas of the surface, which together define the radially outward facing boundary of the plenum. Constant differential pressure means are employed for drawing air through the foraminous laydown surface and the plenum from an air-entrained-fiber deposition chute as the drum is rotated. The specific improvement of the invention comprises stationary adjustable air flow modulating means (shutter plates) disposed adjacent the radially inwardly disposed boundary of an arcuate portion of the plenum circumferentially spanning a plurality of the transverse plenum segments. In such manner, the pressure across the relatively widely masked transverse sections of the laydown surface can be adjusted without substantially affecting the pressure across the relatively narrow masked transverse sections of the laydown surface.
This apparatus purportedly permits the formation of a fibrous web severable into uniform, contoured articles, such as fibrous absorbent cores for disposable diapers, having relatively thick narrow and adsorbent crotch areas, and relatively thin waistband regions, without stepwise basis weight gradients. Thus, the areas of the foraminous laydown surface (screen) having the largest pressure differential across them (i.e., the narrow areas) experience greater fiber buildups or accumulations than the areas of the screen having lower pressure differentials across them (i.e., the wide areas), the narrow areas corresponding to the crotch regions of the web articles and the large areas corresponding to the waistband regions thereof.
Although the apparatus disclosed in the -056 patent is alleged to provide a smooth basis weight gradation in the machine direction (i.e., longitudinally) in the fibrous articles formed thereon, as noted particularly with reference to FIGS. 7 and 8 of this patent, the basis weight gradation of the fibrous article is both longitudinally and laterally symmetrical in distribution. Accordingly, the greatest basis weight occurs in a circular shaped region centered at the crotch with the basis weight uniformly radially decreasing therefrom, such that lines of nominally equal basis weights describe concentric circles radiating outwardly from such central region of highest basis weight. This design provides a high basis weight in the frontal crotch region where same is usefully employed. However, for reasons of liquid retention, it is more advantageous to provide a longitudinally extending central region of high basis weight relative to the longitudinal peripheral margins, and further to provide a higher basis weight in the front panel of the fibrous article relative to its rear panel (the front and rear panels being considered here as the demarcated opposed symmetrical portions produced when the fibrous article is folded along a lateral fold line midway along its longitudinal extent). In comparison to these optimal basis weight characteristics of the fibrous web article, the fibrous articles produced by the apparatus of the Lee, et al. patent are seen to be deficient, particularly in the steady decline of such article's basis weight along the full longitudinal dimension of the front and rear panels, from a point centered at the crotch region of the article.
U.S. Pat. No. 3,939,240 to P. P. Savich discloses a method of dry forming fibrous pads by means of a condenser roll having three-dimensional cavities circumferentially disposed about the periphery thereof. The cavities each have foraminous bottom and side surfaces, with the surface area of the cavity being greater than the surface area of the opening into the cavity. Vacuum is applied through the foraminous surfaces of each cavity to pull the air component of a fibers/air suspension through the foraminous surfaces, thereby depositing the fibers carried in the air suspension onto the cavity surfaces. A transfer conveyor is proximately disposed to the cavity opening at its discharge position and vacuum also is supplied through the transfer conveyor, to transfer the fibrous layer from the cavity onto the conveyor. The fibers deposited on the transfer conveyor are confined to an area substantially equal to the surface area of the cavity opening, so that the fibers are consolidated as they are transferred from the cavity onto the conveyor, forming fibrous pads having a greater basis weight than the basis weight of the fibrous layers formed in the cavities. This patent, in addition to embodiments disclosing the formation of discrete fibrous pads unassociated with any fibrous web, discloses an embodiment in which the outer periphery of the forming roll surrounding each cavity is also foraminous. In such manner, the fibrous pads formed from the fibrous layers within each cavity will be integrally joined with fibrous web sections of a lower basis weight.
U.S. Pat. No. 4,223,677 to J. E. Anderson discloses an absorbent fibrous structure which includes intermingled absorbent fibers of a varying length up to about 6.35 millimeters. The fibers are disposed in different classified layers having differing weighted average fiber length in each of the layers, with the weighted average fiber length decreasing from layer to layer in a direction from one outer surface to the opposite outer surface. The separate layers of the absorbent pad are not disclosed as having any varying weight within the respective layers, so that the basis weight therefore is constant along the longitudinal and transverse dimensions of the pad.
An apparatus for forming fibrous pads is disclosed in U.S. Pat. No. 3,973,291 to C. G. Kolbach, comprising a pad assembly having spaced three-dimensional pad-receiving compartments, separated by air-impermeable regions. The pad-receiving compartments are defined by lower air-permeable surfaces and air-impermeable side walls extending outwardly therefrom. The side wall sections of each compartment are movable relative to the lower air-permeable surface to assist in releasing formed pads from the compartments.
In column 8, lines 41 et seq, it is alleged that the patentee has discovered that formation of a profiled fibrous pad, i.e., one with a varying basis weight, "cannot be controlled within close tolerances by establishing a different amount of open area through which air can be drawn by a vacuum box through different predetermined regions underlying the different predetermined sections of the pad-receiving compartment in which different weights of fibers per unit area are to be deposited". Based on this discovery, it is contended that "the only effective means for establishing different weights of fiber per unit area in different predetermined regions of a fibrous pad, while maintaining close tolerances, is to completely form each predetermined region with a specific weight of fibers per unit area therein substantially independently of the formation of every other predetermined region having a different weight of fibers per unit area therein". The disclosed method thus involves completely masking off a source of vacuum to all sections of each pad-receiving compartment except the section in which region a fibrous pad having a particular weight of fibers per unit area is to be formed. After this region has been completely formed, the vacuum source underlying the formed region is completely masked to the passage of air, and a second section of each pad-receiving compartment is exposed to vacuum to form a predetermined region on the pad having a different weight of fibers per unit area therein. Thus, the patentee discloses a serial masking-unmasking sequence to provide the finished article.
U.S. Pat. No. 3,501,813 to C. A. Lee, et al. discloses a method and apparatus for forming a single integral web of air-laid fibrous material with non-uniform cross-sectional thickness. The disclosed apparatus employs a carrier moving at uniform rate whereon first and second quantities per unit time of loose fibrous material are conveyed by air and deposited on first and second different portions of the carrier. The uneven distribution of material on the foraminous carrier is achieved by providing the air stream with a velocity profile in which certain portions have a higher velocity than the adjoining portions and convey a greater amount of material to associated portions of the carrier, by creating a greater vacuum or suction behind the associated portions of the foraminous carrier than behind adjacent portions so as to draw the air through the screen at the same rate as it arrives at the screen. This in turn is achieved by shielding a portion of the carrier from the air stream while deflecting the air stream toward the unshielded portion.
As shown in FIG. 5 of the -813, Lee, et al. patent, baffles 46 are provided to constrict the conduit through which air is delivered to the carrier, with valves 48 being provided to permit the establishment of a lower pressure behind certain portions of the carrier than behind adjacent portions. The baffles and valves are both selectively operable to provide the web with a predetermined profile or cross-sectional configuration. The web produced by such apparatus, as disclosed in column 4, lines 1-3, has a raised or thick center portion flanked by substantially thinner edge portions. Thus, baffles are provided upstream of the forming surface and valves downstream from the forming surface, with respect to the path of the air flow therethrough. Each of the baffles is in the form of a flat plate beveled at its innermost end and positioned in a slot defined by flanges, which by virtue of the inclination of the flanges, cause the baffles to extend inwardly from opposite sides of the central conduit and be inclined in the direction of flow of the air stream. Accordingly, the baffles constrict the conduit within the delivery duct to narrow the air stream to a centrally disposed vertically oriented flow and thereby increase velocity of the air stream in the central area of the conduit.
The foraminous carrier is supported by a vacuum box which also serves to control the passage of air through the carrier. The vacuum box includes a grid plate provided with a plurality of openings which affords communication between the vacuum chamber and the surface of the grid plate, thereby creating a section which removes air arriving at the surface of the plate. Each opening in the grid plate has associated therewith a valve which permits selective control of air pressure at each opening and consequently, permits variations in the degree of suction across the grid. The web formed on the carrier screen is removed therefrom by a take-off roll associated with nozzles proximate thereto which direct jets of air outwardly through the carrier screen thereby assisting in the separation of the web from the screen, and cleaning the screen of adhering fibrous particles.
U.S. Pat. No. 3,598,680 to C. A. Lee discloses a tandem air former for forming a fibrous web of non-uniform cross-sectional thickness, by air-laying fibrous material at a first station and then air-laying additional fibrous material at a second station downstream from the first to overlap at least partially the fibrous material deposited at the first station. A pressure differential is maintained across the web during formation, to cause air to flow through the thicker portion of the web as well as the thinner portions at substantially the same rate as it approaches the web. The air flow passageway to the foraminous carrier is defined in part by deckle plates which are adjustable transversely of the web being formed, i.e., the respective opposed deckle plates may be shifted toward or away from one another to vary the width of the air stream passing between them and, in consequence, the width of the pad being deposited on the web.
U.S. Pat. No. 3,975,222 to F. K. Mesek discloses a disposable diaper assembly comprising an absorbent fibrous panel which is double contoured, being centrally contoured in the transverse and longitudinal directions to produce a smooth peak on one major surface. Two rolls of compacted wood are provided to feed a source of short cellulosic fibers to a grinding mill from which a stream of fibers is blown downwardly through a duct onto a belt as a layer. The patent discloses that the duct may be baffled to allow more fibers to be concentrated at the central portion of the web. Another method comprises grinding fibers at one station and depositing them to produce a continuous web at the maximum width desired and grinding fibers at another station and depositing them downstream along a band of lesser width on top of and along the median of the first continuous web. A longitudinal contour of the fibrous web is achieved by varying the speed at which fibers are deposited on the belt, so that by decreasing the deposition rate the marginal areas of reduced thickness are produced and correspondingly, by increasing the deposition rate, the thickened central contour portion is produced. The contour thickness is preferably formed to provide a ratio of apex thickness to corner thickness in the range of 1.5 to 4.
U.S. Pat. No. 3,994,047 to C. A. Lee, et al. discloses apparatus for making two-layer composite pads formed simultaneously on a twin wire arrangement, with the units of one layer being of hourglass and the other, ovate in shape. The layers are formed on respective foraminous carriers in a forming chamber. In the disclosed system, it is necessary to keep the respective forming screens in register with one another inasmuch as the webs formed thereon are subsequently joined to form the aforementioned composite. For such purpose, the respective forming screens have registration indicia which may be sensed as for example by an electric eye, to indicate any misregistration whereby the appropriate tension roll for the respective forming screen is adjusted to maintain registration. In order to drive air through the laydown fibrous web layers at the same rate at different portions of the forming path, the pressure differential and the respective forming layers increase in the direction of travel of the carrier screens, by separately controlled air flow through suction boxes associated therewith. Each of the suction boxes includes a damper for controlling the rate of flow of air through each of the boxes. The forming chamber also has a perforated wall opening through which additional air may be admitted to the forming chamber.
A removal means is provided at the exit end of the forming chamber to remove any excess fibers as deposited on the respective carriers. This removal means includes a snout 110 which in turn includes a septum and walls defining openings through which air is sucked by a blower at relatively high velocity. The walls of the snout are disposed relatively close to the tops of formed layers of fibers to provide a rush of air over the exposed surfaces thereof. This shears fibers from the surfaces of the layers and entrains the fibers in the air stream removed therefrom. The foraminous carrier screens in this system include open areas on which the respective fibrous web components are formed, the areas outside of such patterned open areas being impervious (impermeable) to air flow.
U.S. Pat. No. 4,016,628 to C. G. Kolbach discloses an apparatus for forming a fibrous web which includes a medial portion integrally joined through the randomly arranged fibers thereof to flanking side portions and flanking end portions, the medial portion having a greater basis weight and thickness than the respective flanking side and end portions. The patent discloses at column 3, lines 29-40 that the higher basis weight medial portion of the fibrous web can be substantially uniform in basis weight or can be profiled, e.g., with the center section of the medial portion being provided with a greater basis weight of fibers than the flanking end sections (for use as disposable diapers for girls, and alternatively a forward section of the medial portion being provided with a greater basis weight of fibers therein than a rearward section thereof, as when the fibrous web is used as a disposable diaper for boys). In addition, it is disclosed at column 3, lines 41-47 that the specific shape of the medial portion can be varied within wide limits, such as being substantially rectangular or contoured to include a reduced width crotch region which provides a more conformable structure in the perineal region of a wearer.
Embodiments of the product fiber web are shown in FIGS. 11-16 of the Kolbach patent, wherein the medial portion of the web is profiled to itself to have different basis weights in different predetermined sections thereof, such as the medial portion having a center section of greater basis weight than the adjoining end sections of the medial portion (FIGS. 11, 13 and 15) and a configuration wherein a greater basis weight section of the medial portion is provided on the forward half thereof (FIGS. 12 and 16).
The disclosed apparatus employs a foraminous forming surface and at least one vacuum box under a discrete section of the forming surface. The foraminous forming surface and the vacuum box are moved in registration with each other through a web forming area so that the same region of the foraminous forming surface is always in overlying relationship to the vacuum box. In operation, an air suspension of fibers is directed onto the surface of a condenser roll assembly, having a foraminous forming surface disposed thereon with circumferentially spaced, three-dimensional compartments therein. Downstream therefrom may be disposed embossing rolls having corresponding surface recesses therein for embossing the fibrous web, to provide surface contours thereon which channel liquids so that the full absorbent capability of the fibrous web is utilized.
The condenser roll assembly includes an air-pervious condenser roll including a perforated cylindrical metal shell and a porous screen secured about the periphery of the shell. The three-dimensional compartments are established by providing discrete cutout regions in the porous screen and cylindrical metal shell and securing a foraminous member to the lower surface of the shell to bridge each cutout region and thereby form the bottom wall of the three-dimensional compartment. In this manner, the porous screen outside the three-dimensional compartments and the foraminous member constituting the lower surface thereof together provide a forming surface for the condenser roll. A vacuum box assembly is mounted within the condenser roll secured to a driving axle so as to be concurrently rotated with the cylindrical shell. The vacuum box assembly includes a plurality of circumferentially spaced vacuum boxes attached to a cylindrical hub, with the outer edges of the sidewalls of the vacuum boxes defining an opening into the vacuum box of the same shape as the three-dimensional compartments, when aligned therewith. Since the cylindrical shell and the vacuum box assembly are concurrently rotated at the same angular velocity, each vacuum box will underlie its respective compartment throughout the entire path of rotation. Removal of the formed web from the condenser roll is effected by terminating the partial vacuum through the forming surface; this is achieved by a masking member circumferentially extending in coaxial manner with the condensing roll and the vacuum box assembly, and interposed therebetween. The masking member is perforated over a portion of its length and is imperforate on the lower section.
Accordingly, the condenser roll and vacuum box assembly during its travel will encounter the imperforate portion of the masking member, thereby blocking the vacuum imparted to the formed web, so that the web may be removed by a take-off conveyor to which vacuum is applied. The masking member is not rotated, so that the condenser roll and vacuum box assembly pass circumferentially adjacent to the masking member over its full circumferential extent. Vacuum is provided to the forming surface by suction through an annular passage in flow communication with the vacuum boxes. The portions of the interior of the condenser roll are in flow communication with a second annular passage with a reduced vacuum level being imposed thereon. The patent states at column 12, lines 56-62 that since the entire forming surface of the condenser roll is exposed to vacuum for the same period of time, a greater effective or total volumetric air flow will be established through the bottom walls of the three-dimensional compartments, which are subject to a greater partial vacuum than the surfaces flanking the three-dimensional compartments. This greater effective volumetric air flow results in the deposition of a greater weight of fibers in each of the three-dimensional compartments than on the foraminous surface regions surrounding the three-dimensional compartments. The drawings of this apparatus, such as FIG. 3, indicate the three-dimensional compartment to be relatively shallow and to be bounded by radially extending wall surfaces which appear to be perpendicular to the forming surface.
In a modified embodiment of the invention, as shown in FIGS. 17-19 of the patent, a masking member is employed in which the imperforate circumferential portion has at one end thereof "finger portions" which underlie the three-dimensional compartments at their extremities; these finger portions are followed circumferentially by a central blocking portion of the mask member which in turn underlies the central portion of the three-dimensional compartment when same is passed over the masking member. The circumferential extent of the finger portions is greater than the circumferential extent of the central blocking portion of the masking member, whereby the central region of the three-dimensional compartment will be exposed for a greater period of time to the vacuum, to achieve formation of thickened central portions of the three-dimensional compartments.
Finally, the patent states at column 15, lines 2-7 that "it is within the scope of this invention to provide a fluid impervious coating directly on the forming surface of the condenser roll in the form of a transverse stripe disposed intermediate adjacent three-dimensional compartments to achieve direct formation of discrete fibrous webs on the forming surface". It is apparent that this coating serves as a fiber deposition blocking means to segment the web during laydown, in discrete segments.
U.S. Pat. No. 3,518,726 to C. T. Banks discloses apparatus for making sanitary napkins from fluff derived from wood pulp which has been disintegrated. A forming drum is employed which has on its cylindrical surface a series of planar chord-like plate members which are perforated, each plate having perforations more closely spaced relative to one another in the central portion of the plate as compared to the end segments of the chord-like plates, wherein the perforations are more distantly spaced relative to one another. The forming drum is in gas flow communication with vacuum suction means, whereby areas of fluff deposited on the drum compartments are of greater thickness at their centers relative to their ends, due to the arcuate shape of the side plates defining each compartments on the drum periphery. In addition, more fluff collects in the central regions of the cavities than in the end regions due to the fact that the spacing between holes is less in the central regions than in the end regions, so that the resulting pad is thicker in its central region.
This patent also discloses an embodiment wherein a compressed, high-density segment is provided for embedment in one of the flat faces of the pad. In this embodiment, a second assembly takes disintegrated pulp fibers and collects some on a forming drum which is composed of a cylinder of perforated sheet material, with a vacuum box in communication with a central opening through the rotatable drum for maintaining vacuum therein. An end closure plate portion extends across and within the forming drum, being sealed with respect to the inner surface thereof. In this fashion, only about one-half of the forming drum has suction applied to it. This forming drum is in close-spaced relationship to a similar forming drum, whereby each collects fluff on its exterior surfaces for discharge therefrom between the drums in the form of a continuous batt. The batt then is debulked between compression rolls and indented at spaced intervals, following which the batt passes through an embossing assembly which provides a pattern of pyramidal-shaped depressions therein. The batt then is cut into segments and transmitted by conveyor means to the first-mentioned forming drum, on which the high-density segment is centrally disposed on the forming cavity and overlaid with fluff. The resulting pads,, containing the high-density segments on their lower faces, are cut in a cutter assembly and passed to a wrapping means whereon a gauze web is folded around the individual pads, pleated and severed to form the individual sanitary napkin.
As is apparent from the foregoing, the prior art has proposed a great variety of systems for producing variant basis weight articles by air-laying of fibers, and has proposed numerous means of removing the laid fibrous article from the forming surface, yet all are characterizable to some degree by deficiencies in terms of the mechanical complexity involved as well as in deficiencies of the absorbent material which is produced.
Insofar as the latter point, i.e., the character of the absorbing material produced by laying of the fibrous material, is concerned, it is highly desirable in applications such as the production of absorbent pads for disposable diapers to provide a non-stepwise gradation of basis weight, with the highest basis weight being provided forward of the lateral center line (crotch fold line) of the article, as compared to the rear portion thereof and to provide in the central region along the length of the article a higher basis weight than the peripheral or marginal areas, and further to provide a still higher basis weight in the central frontal region of the article.